That kind of evaporation is not uncommon. However, when adding water, don't worry about putting the Amquel into the water if you prepare water properly. Most of the top brand salts on the market remove chlorine instantly, so buying a product to do this is really pointless. Depending on the type of salt that you use however, you should really prepare it 24 hours in advance and let the salt dissolve. For example, if you use Instant Ocean, the salt dissolves in 10 minutes. If you are using the cheaper Crystal Sea, it takes 24 hours. You should never put undissolved salt water into your tank unless it is an emergency. Most experienced marine hobbyists prepare their top-off water a day in advance, unless you are using the better salt brands, such as Instant Ocean.
I'm not saying that there's anything wrong with Crystal Sea, the only negative is that it takes 24 hours for the salt to fully dissolve in the water.